An image forming apparatus refers to an apparatus configured to print an image on a printing medium in accordance with a received signal. An image forming apparatus can be classified as, for example, a printer, a copying machine, a fax machine, or a multi-function printer that can provide several functions such as printing, scanning, copying, faxing, and the like.
An electro-photographic type image forming apparatus is a kind of image forming apparatus in which light is scanned on a surface of a photosensitive member charged to a predetermined electric potential to form an electrostatic latent image on the surface of the photosensitive body. A toner can be used as a developer and can be supplied to the electrostatic latent image to form a visible toner image. The visible toner image formed on the photosensitive member can be transferred to a printing medium directly or via an intermediate transfer member. The image transferred to the printing medium can be fused to the printing medium through a fusing process.
Generally, an electro-photographic type image forming apparatus can include a cleaning device to remove residual toner from the surface of the photosensitive member by mechanically scraping the residual toner. Removing contaminants (e.g., residual toner, paper dust) from the surface of the photosensitive member by using a cleaning device can prevent a charge roller of the electro-photographic type image forming apparatus from being contaminated.
Some electro-photographic type image forming apparatuses have been recently developed that do not include a cleaning device. Such apparatuses can achieve a compact size, convenience in manufacturing, and cost savings. Such apparatuses employ a method of interrupting power applied to a charge roller and to a developing roller after completing image formation to decrease contamination of a charge roller from contaminants (e.g., residual toner, paper dust) on the surface of the photosensitive member.
By way of an example, an electro-photographic type image forming apparatus (hereinafter, referred to as “image forming apparatus” for brevity) without a cleaning device can be operated such that the photosensitive member is charged using a charge roller to which is applied a charging voltage of the same polarity as the polarity of the charged toner. Light representative of the desired image is then scanned on the surface of the charged photosensitive member by using a laser scanning unit (LSU) as an exposure unit to form an electrostatic latent image. Negatively charged toner can be coated on the photosensitive member using a developing roller by applying a negative voltage (e.g., a voltage of the same polarity as the toner) to the developing roller. The toner can be transferred to the electrostatic latent image on the photosensitive member at a nip formed between the photosensitive member and the developing roller. To transfer the toner from the surface of the photosensitive member further onto paper, a positive voltage (e.g., a voltage of opposite polarity to the polarity of the toner) can be applied to a transfer roller. The toner transferred to the paper remains attached to the paper by electrostatic force, and, when the paper is made to pass through a fusing unit, can be permanently fused to the paper by heat and pressure produced by the fusing unit. Then, the paper is discharged to a space outside the image forming apparatus through a paper discharge unit.
When the toner has a positive polarity, voltages of the opposite polarity to the afore-mentioned voltage can be applied. In other words, a positive voltage can be applied to the developing roller and a negative voltage can be applied to the transfer roller. Thus, generally speaking, a voltage of the same polarity as the polarity of the charged toner can be applied for the developing process while a voltage of an opposite polarity to the polarity of the charged toner can be applied for the transfer process.
An image forming apparatus can perform a cleaning operation in such a manner that when the movement of the paper away from the fusing unit is detected by a discharge sensor disposed, e.g., in the back-end portion of the fusing unit, the voltage applied to the charge roller and to the developing roller can be interrupted to prevent toner of an opposite polarity (e.g., positively charged toner particles) from being attached to the charge roller. The charge roller, however, can nevertheless be contaminated by paper dust and/or toner from multiple image forming operations. When the charge roller becomes contaminated, the charging performance of the photosensitive member can deteriorate, and may result, in poor image quality or even in image defects.
The contamination of the charge roller or of the transfer roller by the residual toner on the photosensitive member can adversely impact the charging operations and/or result in the contamination of the back surface of the paper during the transfer process. Furthermore, a degradation of image quality in the form of a background image forming during the developing process may also occur because the toner on a non-image area is not properly collected.